Process for the production of cyanogen halide and apparatus for use therewith



Jan. 24, 1967 I J. w. SPRAGUE 3,300,398

PROCESS FOR THE PRODUCTION OF CYANOGEN HALIDE AND APPARATUS FOR USE THEREWITH Filed Oct. 12. 1962 H NH United StatesPatent O 3,300 398 PROCESS FOR THE PRODUCTION OF CYANOGEN AND APPARATUS FOR USE THERE- James W. Sprague, Streetsboro, Ohio, assignor to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio Filed Oct. 12, 1962, Ser. No. 230,131 12 Claims. (Cl. 204101) This invention relates to a process for the production of a cyanogen halide and to an apparatus for use therewith. More particularly, the present invention relates to a process for the production of a cyanogen halide by the electrochemical reaction of an ammonium halide with hydrogen cyanide and to an improved electrochemical cell in which said process can be conducted.

The invention relates to an electrochemical cell of the diveded type, wherein an ion-permeable and gas-permeable membrane is employed to divide the cell into two compartments, a cathode compartment and an anode compartment. a cell of the divided type and to an electrochemical process conducted in said cell wherein a cyanogen halide is produced in the anode compartment by the reaction of hydrogen cyanide and an ammonium halide, which may be either the bromide or the chloride. In such a process and apparatus, gaseous ammonia and gaseous hydrogen are both formed in the cathode compartment and free bromine or chlorine is formed, as a side product, in solution in the anode compartment.

The invention is rnost particularly related to i I This invention is an improvement in both the process and the apparatus which are set forth in my copending application, Serial No. 162,333, now Patent No. 3,168,- 458, filed December 27, 1961, inasmuch as that applicationapplies to the production of a cyanogen halide.

In my copending application Serial No. 162,333 as it relates to the present invention, an electrochemical cell of the divided type is set forth in which a perforated cathode, adapted to transfer liquid from one side of the cathode to the other, is in combination with electrolyte absorbing means disposed on one side, which means is gas permeable and at the same time capable of maintaining an interfacial contact between the perforated cathode and the electrolyte and which provides a continuous internal circuit for the transport of ions through both compartments of the cell. Briefly, such a cell includes a perforated cathode, preferably of regular geometric configuration, having opposed marginal boundaries and forming one wall of a cathode compartment. At one marginal boundary there is provided means for supplying catholyte to one surface of the cathode, and at an opposing marginal boundary there is provided means for collecting catholyte which has been replaced by fresh catholyte, as well as any excess catholyte in the cathode compartment. Thus, the catholyte in the cathode compartment is restricted to the surface of .the cathode and the bottom of the cell from the cathode to the outlet. The-perforated cathode serves as one wall of the cathode compartment and it and the gas-permeable, electrolyte absorbing means adjacent thereto, divide the cathode compartment from the anode compartment of the cell, One wall of the anode compartment comprises an impervious anode. Catholyte and anolyte are kept separate by an ion-permeable membrane, which also is a part of the divider, and can be located adjacent the electrolyte absorbing means. Such a cell is particularly well adapted for electrochemical conversions in which one or more gases is evolved and, thus, has been employed for the synthesis of cyanogen bro mide or cyanogen chloride from hydrogen cyanide and ammonium bromide or ammonium chloride. Such a reaction is illustrated by the following equation:

ICC

vided wherein X is Br or Cl. When the reaction illustrated above is conducted in a cell of the divided type, both the anodev compartmentandthe cathode compartment contain NH Br or NH Cl as the electrolyte. HCN and .NH Br or NH Cl and water are fed to the anode compartment. CNBr or CNCl is is produced in the anode compartment and is recovered therefrom. H gasand NH gas are produced in the cathode compartment, where there is ample reserve space to receive them, and are recovered therefrom. In addition, the use of a gas-permeable electrolyte absorbing separator between the perforated cathode and the ionapermeable membrane, results 1 in prolonged membrane life and improved conductivity.

The inventionprovides .a cell wherein the electrolyte in the anode compartment'can be maintained at a pressure substantially in excess of the pressure of electrolyte in the cathode compartment. This type of cell has the following advantages: It requires reduced operating voltage; it provides high conductivity, and it results in high conversion rates. A

Such a cell and process have demonstrated that excellent yields and long electrolyte life can be obtained employing commercially available ion-permselective membranes to divide the cells However, in certain instances it has been observed that a degradation of the lower portion of such membranes occurs.

It is believed that this is due to chemical reactions within the membrane of halogen gas, formed as a result of a side reaction in the anode compartment, Halogen gas can enter the membrane. Ammonia which is formed in the cathode compartment may also be present in the membrance. The resulting reactions are probably complex reactions involving bromine (or chlorine), ammonia (if present) and the materials of which the membrane itself is composed (for example, quaternary ammonium groups). These reactions result in deterioration of the membrane.

The present invention comprises a novel process and an improved electrochemical cell, particularly adapted for the synthesis of cyanogen bromide or cyanogen chloride from hydrogen cyanide and ammonium bromide or ammonium chloride.

The process of the present invention makes it possible to revent free gas Which forms in solution in the anode compartment from entering and, thereafter, reacting in the membrane. This is accomplished by reacting free halogen in solution with hydrogen cyanide, and establishing a flow within the cell so as to withdraw free halogen for such reaction before it can diffuse into the membrane.

The reaction of hydrogen cyanide with bromine or chlorine results in the production of hydrogen bromide or chloride and cyanogen bromide or chloride. If an excess of hydrogen cyanide is'introduced into the anolyte, more than that required to react with the halogen in the anolyte, and the anolyte then passed upwardly over the surface of the membrane, the hydrogen cyanide in the solution passing up over the membrane will react with any halogen which migrates toward the membrane, so that substantially no halogen reaches the membrane. Thus, introduction of a slight excess of hydrogen cyanide into the anolyte is preferred.

A modification of the present invention involves withdrawal ofthe halogen-rich phase from the base of the well, mixing it with fresh hydrogen cyanide-anolyte feed, whereby the halogen and'hydrogen cyanide react to form hydrogen halide and ammonium halide, and returning the material to the cell at a point above the top of the well.

. This modification has the advantage that the well can be made as small as possible to achieve minimum capital investment and/or to fi-t in a smaller working space.

Thus, the present invention comprises a process for the manufacture of a cyanogen halide comprising sub 'ecting a solution of hydrogen cyanide and an ammonium halide selected from the group consisting of ammonium bromide and ammonium chloride to the action of a direct electric current in an electrolytic cell in which the cathode and anode compartments are divided by an ionpermeable membrane, withdrawing a halogen-rich phase from the anode compartment, admixing said phase with hydrogen cyanide in an amount at least sufficient to react with all of the halogen present in said phase, and returning the resulting solution to the anode compartment. Preferably, a slight excess of hydrogen cyanide is employed, and preferably the reaction with hydrogen cyanide is conducted in a well located immediately beneath the anode compartment. Furthermore, the solution resulting from the reaction of halogen with hydrogen cyanide is preferably passed up over the surface of the ion-permeable membrane, to serve as a supplemental barrier to diffusion of halogen therewithin.

In one embodiment of cell designed to carry out this process, a separate zone or well is provided at the bottom of the anode compartment of the cell, to take advantage of the increase in specific gravity of the ammonium halide solution after halide ion has been discharged at the surface of the anode. Therefore, a downward flow of highly concentrated aqueous ammonium halide solution containing free halogen runs along the face of the anode, and is collected in the well at the bottom of the cell where it is reacted with fresh hydrogen cyanide introduced at or near the bottom of the well.

The general structure of the cell of this invention is similar to that of Serial No. 162,333. The perforated cathode, as in that cell, preferably has guide portions coacting with the perforations. The guide portions may be of such configuration that they form scoops, either regularly or randomly disposed from the surface, to intercept liquid trickling down over the surface and to guide it from one surface to the opposite surface through the perforations thereof. The upper boundary of the cathode is provided with means which permit catholyte to be recycled after trickling down over the surface of the cathode.

While simple metal surfaces such as stainless steel, have been found quite satisfactory for use as the perforated cathode,the surface of metallic material which is in contact with the electrolyte absorbing separator may be activated by deposition thereon of reactive or catalytic materials and surface-multiplying agents such as platinum black, nickel black, and various metal oxides, etc. Metals of low hydrogen overvoltage are preferred.

A useful criterion in perforated electrode design is the ratio of the frontal electrode area excluding perforations, to the perforation diameter, which should be between about and 25. The percent open area may range from about 10% to about 30% of the total area. These values are not critical, but are convenient guides in electrode design to achieve the lowest dynamic internal resistance.

As the perforated cathode there can be used a perforated metallic plate or a metallic screen which contains random or regularly spaced openings which will also provide means for transmitting the catholyte from one surface of the cathode to the opposite surface by means of the electrolyte-absorbing separator.

Disposed in contacting laminar relation with the perforated cathode is a gasand liquid-permeable electrolyteabsorbing separator. This may be an inert highly porous material, for example a felt mat, and may be made of such materials as glass fibers, paper fibers, asbestos fibers, animal fibers, vegetable fibers, synthetic fibers or the like. The electrolyte-absorbing separator will soak up the electrolyte transmitted through the perforations of the cathode, and maintain the surface of the cathode opposite the cathode compartment saturated with electrolyte. At the same time, since it becomes saturated with electrolyte, it provides a continuous ion-containing and conducting medium.

The electrolyte-absorbing separator should be chemically stable and should preferably have a liquid permeability of at least about 100 gallons of water per minute per square foot per psi.

The anode should be resistant to the reactants present in the anolyte, and to bromine or chlorine. A variety of materials are available which meet these requirements, such as graphite, carbon, platinum, titanium, stainless steel, etc.

Ion permeable membranes of the type which are employed to separate the anode and cathode compartments are conveniently those membranes which are electrically conductive and permeable to ions, but which are not permeable to molecules, such as the permselective membranes, i.e., those permeable to cations or anions but not both. Hence, they are referred to as cationic or anionic, as the case may be, and both types are useful in accordance with the present invention. An example of a specific ion-exchange resin membrane is as follows:

A mixture of about parts by weight of styrene and about 5 parts by weight divinyl benzene was polymerized. The resulting polymer was comminuted t-o fine particles and parts by weight of this finely-divided material was sulfonated by reaction with about parts by weight of chlorosulfonic acid. The latter reaction was carried out by heating at reflux temperature for about 3 minutes and then maintaining the mixture at room temperature for 50 hours. The sulfonated product was then washed with a large excess of water to remove any remaining chlorosulfonic acids and any acid chlorides which were formed in the reaction. The sulfonated resin was then dried and 2 parts by weight of the dried resin were mixed with 1 part by weight of polyethylene and the resulting mixture was pressed into a sheet which then serves as the membrane.

The preparation and description of permselective membranes is well known in the art and there are numerous patents relating to such membranes. Examples of such membranes are described in US. Patents Nos. 2,636,851, 2,636,852, 2,861,319, 2,861,320, 2,702,272, 2,730,768, 2,731,403, 2,731,411, 2,731,425, 2,732,351, 2,756,202, 2,780,604, 2,800,445, 2,820,756, 2,827,426, 2,858,264, 2,860,096, 2,860,097, 2,867,575, 2,894,289, 2,903,406, and 2,957,206. Any of the membranes disclosed in the patents in the foregoing list may be employed in constructing the apparatus of the present invention.

As for the conditions under which the cell is conveniently operated, it has been found that currents in the range of from 10 to 1,000 amperes per square foot are satisfactory and the preferred range is 30 to 500 amps per square foot. The cell may be operated at voltages in the range of 2 to 6 volts, and the preferred voltage is in the range of from about 3 to about 4. In the electrochemical synthesis of cyanogen halide from hydrogen cyanide, the actual voltage will be determined by the current density. 1

The preferred temperature of operation is in the range of 20 to 75 C., it being understood that certain of the membranes employed as barriers are quite sensitive to temperature, and consequently the temperature at which the reaction is carried out will be below that at which the membrane is disadvantageously alfected. Most membranes exhibit quite satisfactory physical stability at tempgr atcures within the preferred range, namely, 20 to The product of the electrochemical reaction is found 1n the anolyte and may be removed by any convenient chemical or physical means, such as distillation.

The positive pressure exerted by the head of the anolyte 1n the anode compartment as a result of the essentially empty cathode compartment holds the membrane firmly and evenly in place against the electrolyte-absorbing separator, which in turn is supported by the cathode. This prevents lateral motion of the membrane, and minimizes membrane wear. The pressure in the cathode compartcell, at the boundary between the anode compartment 3 and cathode compartment 8. Cathode 4 is in contact with the electrolyte-absorbing separator 9 which in this case is made of a sheet of wool felt, and this in turn is ment can be varied to adjust the pressure differential be- 5 in contact with membrane 10, which can be of permtween the cathode and anode compartments. Vacuum selective material. Well 11 is an extension of anode commay be applied to the cathode compartment in order to partment 3, into which the anode extends. Inlet port 12 aid in gas removal. The decrease in pressure in going and outlet port 13 are provided in anode compartment 3, from the cathode to the anode compartment must be and inlet port 14 and outlet port 15 are provided in greater than the pressure drop of the gas going through cathode compartment 8. Anolyte is withdrawn through the cathode. Generally, less than about two pounds per port 13 in recovery of cyanogen halide, and hydrogen square inch differential is sufiicient. Maximum pressure cyanide together with fresh and recycle anolyte to reis limited by the strength of the cell barriers. place that withdrawn at 13 are introduced into compart- Operable pressures are dependent upon the degree of ment 3 through port'lz, for reaction with halogen in the porosity and size of pores of the cathode and the current Well catholyte is Withdrawn from cathode p density, which controls the amount of gas produced at ment 8 through P y means of line and is the cathode surface. The inter pore distances may be cycled to compartment 8 by means of line 19 through increased at the expense of an increased pressure differen- P Gas is removed from t Cathode ompartment tial. With small inter pore distances, there is more rapid 8 through P and g lines 18 and gas clearance. The use of wire gauze as the cathode has 20 In p anolyte circulates through anode been found to be particularly suitable from this standpartment 3 entering the compartment from line 16 point, because of the rapid gas removal, its effective surthrough port 12, blending with anolyte passing downface and sufficient contact. With too small a pore size WaIdly Over anode 2 into Well 11, and then Passing pthere is an excessive rate of gas accumulation, and im- Wafdiy P membrane 10 towards Well 11, at Which a practically high pressures across the cell barrier are re- Portion is bled out through P and line quired to force the gas through the perforations and out Y from Which cyanogen halide has been recovered is of the cell. The resultant effect of this cell structure recycled through line 16 to the anode compartment, and creates high power efficiencies. With th use f h HCN is introduced to it through line 5 before it enters brane, and electrolyte-absorbing separator, interelectrode the compartmentdistances less than those normally encountered may be v Cathoiyte is introduced at the p of the Cathode utilized, sothat advantage can be taken of the resulting mp r m n 8 .in u h a mann r that it trickles over decrease in cell voltage and in the overall size of the cathode The catholyte is removed as fa a i 001- u it Di t as ho t a f an i h may be lects at the bottom of the compartment through port 15 ployed without danger form an internal short circuit. and line Gases formed in compartment 8 are also In addition, the pressure differential across the barrier removed through P 15 and line The gases, y to the gas producing electrode permits the accumulation g and ammonia are removed from line 18 through line f 1 a minimum amount f gas between h b i 6, and the catholyte is then recycled to the. cathode comand the perforated electrode. Since the operating voltpartment 3 thl'oughlilie 19 P t14. age and the accumulation of gas in this space a i t The following examples in the opinion of the inventor proportion, this effect too increases power efficiencies 40 r present p eferred embodiments Of the invention. The overall effect of this cell structure provides the maximum conductivity path between the barrier and the elec- Examples] and 2 trode face or the minimum operating voltage at a given current density, electrolyte concentration and tempera- TWO electmhemlcal reactlons were carved out In the tum cell described 1n FIGURIE 1. The reaction conditions When the anolyte is composed of an aqueous solution and results of these r.eactlons. are Presents? in.Ta.b1e of hydrogen cyanide and ammonium bromide, and the In each of these react ons, with the exceptions ndicated catholyte is a saturated ammonium bromide solution, the below hYdrPgen cyanide and ammmum bromlde products of the reaction at the cathode are ammonia gas electrochemlcany reacted to t 9 bromide and hydrogen, which are removed with the catholyte An solutlon of iammomlum bromlde and hydro through the exit ports, and the product removed from the gen was anolyte Into anode compartanolyte is cyanogen bromide. With these coacting elecment of the e Whlch cyanogen :bromlde was formed trolytes, in an electrochemical cell of the invention, under and ,from whlch It recovered- Hydrogen and an impressed voltage of 32 volts at a current density of moma were formed and recovered from the cathode com- 100 amps per square foot, and an anolyte feed rate of 5 partment and both anolyte and catholyte were recycled as 49 ml. per minute, an HCN conversion of 89% to described Connectlon Wlth FIGURE cyanogen bromide and a current ff i of 92% was A flat perforated carbon steel screen A inch d.) was obtained. used as the cathode. In each case, the electrolyte em- The figure is a schematic illustration of an improved P y in both compartments Was q N i electrochemical cell construction in accordance with one containing an equilibrium Concentration of ielntini'ohi'rl embodiment of h presentinvention, of 8 to 10 N, and the interelectrode distance was 1 The figure shows, in diagrammatic form, an ele t o- The table shows the number of faradays required per chemical cell 1 in accordance with the present invention. mole of HCN (F/HCN), current density, the initial po- The cell has an anode 2 as one wall of the anode comtential of the cell, the decay of the cell and the condition partment 3. Cathode 4 is positioned vertically in the of the membrane. TABLE Example Current Initial Decay No. F/HCN Density Potential (volt/day) Membrane Condition (after drying) (amp/ft.) (volts) 1 1. 9 3. 29 0.038 Only trace of polymer, and slight embrittlement, good condition. 2 1.8 200 4. 40 0.057 Only trace of polymer, trace of bubbles,

good condition.

Each of these electrochemical reactions was continued for over five days. At the end of this time, only a trace of brown polymeric material had accumulated in the membrane, and the membrane flexibility was largely retained.

This showed the effect of circulating anolyte so that the bromide was carried into the well and reacted with hydrogen cyanide there before it could reach the membrane.

A cell in which there was no well and in which anolyte was not circulated was operated under similar conditions as were employed in Examples 1 and 2. After five days of operation, the membrane in this cell had to be replaced, since it was found to be brittle and embedded in a polymeric honeycomb which prevented effective operation of the cell.

Obviously, the electrochemical cells of this invention can be employed in conjunction with other equipment. For example, in a commercial installation a multiplicity of such cells could be employed in series or parallel in general configurations which are well known in the art.

Iclairn:

1. A process for the electrochemical production of cyanogen halide using a cell comprising an anode compartment, a cathode compartment, an anode positioned vertically at one side of the anode compartment, and a membrane positioned vertically and separating the two compartments, and positioned at the opposite side of the anode compartment, comprising subjecting an aqueous solution of hydrogen cyanide and ammonium halide to the action of a direct current in the anode compartment so that at the anode halogen is discharged and cyanogen halide formed, flowing heavy halogen-rich anolyte down wardly by gravity at the anode to the bottom of the cell, adding hydrogen cyanide to the halogen-rich anolyte and lowering its specific gravity, flowing the lighter hydrogen cyanide-enriched anolyte upwardly across the anode side of the membrane and thereby preventing halogen in the halogen-rich anolyte from reaching and attacking the membrane, and reacting the halogen therein with hydrogen cyanide to form cyanogen halide, removing cyanogen halide from the anolyte, and recycling the anolyte for further electrochemical reaction.

2. A process in accordance with claim 1 in which hydrogen cyanide is added in excess of that required to react with the halogen in the halogen-rich anolyte.

3. The process of claim 1 in which the halogen-rich anolyte is withdrawn from the anode compartment for reaction with hydrogen cyanide and then recycled to the anode compartment.

4. The process of claim 1 in which the halogen is bromide.

5. The process of claim 1, in which the halogen is chlorine.

6. The process of claim 1 which includes diluting the heavy halogen-rich anolyte with a feed of fresh anolyte and hydrogen cyanide, and flowing the lighter feed-enriched anolyte upwardly across the anode side of the membrane.

7. An electrochemical cell comprising, in combination, an anode compartment and a cathode compartment, a perforated cathode in the cathode compartment, an electrolyte-absorbing separator in contact with the cathode, and a membrane in contact with the separator, separating the anode and cathode compartments, the membrane and separator being positioned vertically and defining one side wall of the anode compartment, an anode positioned vertically and defining another side wall of the anode compartment, a well therebetween at the bottom of the anode compartment, and a horizontally extending portion at one side of the anode compartment defining the top of the well, the membrane being positioned in the said portion as one wall thereof, and means for lowering the specific gravity of anolyte in the well, the space above the wall between the membrane and the anode being open and substantially unobstructed, permitting downward circulation of heavier anolyte at the anode into the well and upward circulation of lighter anolyte from the well across the membrane, thus providing gravity circulation of anolyte Within the anode compartment.

8. An electrochemical cell in accordance with claim 7 in which the membrane is ion-permselective.

9. An electrochemical cell in accordance with claim 8 in which the membrane comprises an ion exchange resln.

10. An electrochemical cell in accordance with claim 7 in which the anode forms one wall of the anode com partment and extends into the well.

11. An electrochemical cell in accordance with claim 7 in which the Well extends from adjacent the anode to adjacent the membrane.

12. An electrochemical cell in accordance with claim 7 in which the means for lowering the specific gravity comprises means for introducing hydrogen cyanide into the well.

References Cited by the Examiner UNITED STATES PATENTS 548,162 10/1895 Hargreaves et al. 204283 641,571 1/1900 Witter 204101 1,152,772 9/1915 Wheeler 204283 1,357,400 11/1920 Jewell 204283 3,069,334 12/1962 Ziegler et al 204263 3,074,863 1/ 1963 Jasionowski 204237 3,105,023 9/1963 Foreman et al 204101 FOREIGN PATENTS 2,660 1895 Great Britain.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

L. G. WISE, H. M. FLOURNOY, Assistant Examiners. 

1. A PROCESS FOR THE ELECTROCHEMICAL PRODUCTION OF CYANOGEN HALIDE USING A CELL COMPRISING AN ANODE COMPARTMENT, A CATHODE COMPARTMENT, AN ANODE POSITIONED VERTICALLY AT ONE SIDE OF THE ANODE, COMPARTMENT, AND A MEMBRANE POSITIONED VERTICALLY AND SEPARATING THE TWO COMPARTMENTS, AND POSITIONED AT THE OPPOSITE SIDE OF THE ANODE COMPARTMENT, COMPRISING SUBJECTING AN AQUEOUS SOLUTION OF HYDROGEN CYANIDE AND AMMONIUM HALIDE TO THE ACTION OF A DIRECT CURRENT IN THE ANODE COMPARTMENT SO THAT AT THE ANODE HALOGEN IS DISCHARGED AND CYANOGEN HALIDE FORMED, FLOWING HEAVY HALOGEN-RICH ANOLYTE DOWNWARDLY BY GRAVITY AT THE ANODE TO THE BOTTOM OF THE CELL, ADDING HYDROGEN CYANIDE TO THE HALOGEN-RICH ANOLYTE AND LOWERING ITS SPECIFIC GRAVITY, FLOWING THE LIGHTER HYDROGEN CYANIDE-ENRICHED ANOLYTE UPWARDLY ACROSS THE ANODE SIDE OF THE MEMBRANE AND THEREBY PREVENTING HALOGEN IN THE HALOGEN-RICH ANOLYTE FROM REACHING AND ATTACKING THE MEMBRANE, AND REACTING THE HALOGEN THEREIN WITH HYDROGEN CYANIDE TO FORM CYANOGEN HALIDE, REMOVING CYANOGEN HALIDE FROM THE ANOLYTE, AND RECYCLING THE ANOLYTE FOR FURTHER ELECTROCHEMICAL REACTION. 